removing all the simple response wrapper classes

while remaining backwards compatibility, this pr get's rid of `SimpleItem` `SimpleCart` and `SimpleNode`.

users relying on the old api can `require 'asin/adapter'` to keep the old api. all tests are still relying on that interface to ensure that it actually works.

using the adapter is considered deprecated and it will be removed in the next version.

/cc @kurtfunai